MODEL ESTIMATES OF HUMERAL HEAD JOINT FORCES USING MEASURED ARM MOTION DURING OCCUPATION AND RECREATION ACTIVITIES

Abstract

An understanding of forces that act on the shoulder joint is important for designing, testing, and evaluating shoulder arthroplasty products. Last year, we presented data describing upper arm motion during eight in-situ hours of occupational and recreational tasks. Using that data the associated humeral head joint forces were calculated with an upper extremity model in OpenSim.

Five subjects from a nonrandom sampling of occupations wore the Inertial Measurment Units during a four hour period while at their work place performing their normal work duties and then during the four hour period of non-work activities immediately following. An unscented Kalman filter (UKF) was used to produce the 3D humeral – thoracic angles at 128 Hz from the IMU data.

Because of the very large number of data points collected with the IMUs, ninety samples of twenty second duration were randomly selected from each four hour collection period. Using the sampled files, the time scales of the sampled files were scaled by a factor of five and then analyzed with the SUEM static optimization and joint reaction features. Not every sample file could be modeled resulting in an average number of sampled files of 66.7 per subject and condition (work/recreation).

The humeral – thoracic angles were then used as input to the Stanford Upper Extremity Model (SUEM) in the OpenSim environment. The SUEM model allowed 2 rotation degree of freedom (rdof) for the forearm (flexion twist), 3 rdof at the humeral – scapular joint, and predicted scapular motion based on the humeral – thoracic elevation angle. All models were run for an assumed 80 kg body weight and included the bone mass of the scapula, clavical, humerus, radius, and ulna, but none of the soft tissue mass. Shoulder muscles were represented by 15 actuators: three heads for each of the deltoid, latissimus dorsi, and pectoralis, and 1 head each for the coracobrachialis, infraspinatous, subscapularis, supraspinatous, termes major, and teres minor.

The 5^th, 50^th, and 95^th percentile values of each force component acting on the humeral head from each sampled file for each subject and condition were calculated and the distribution of forces was plotted as a histogram. The overall mean and standard deviation for the 5^th, 50^th, and 95^th percentile values were also determined.

Of the A-P and S-I force components, anterior and inferior directed force components were larger than the posterior and superior directed force components. For the M-L force component, the forces were nearly exclusively directed in the medial direction. The 5^th and 95^th percentile forces during these subjects' ADL were generally lower than those described by Westerhoff 2009, suggesting that within the limitations of the modeling assumptions, loading experienced during in-situ ADL may be different than loading during laboratory simulation of representative motions.